1. Field
The present invention relates to a display device, and a device and method for driving the same.
2. Related Art
A touchscreen is an input device which is provided to an image display device, such as a liquid crystal display, a field emission display (FED), a plasma display panel (PDP), an electroluminescence device (EL) and an electrophoretic display such that a user inputs predetermined information into the image display device by pressing (or touching) a touch sensor included in the touchscreen while watching the image display device.
A driver circuit of a display device includes a pixel array that displays an image, a data driver circuit that supplies a data signal to data lines of the pixel array, a gate driver circuit (or a scan driver circuit) that sequentially supplies a gate pulse (or a scan pulse) synchronized with the data signal to gate lines (or scan lines) of the pixel array, and a timing controller that controls the data driver circuit and the gate driver circuit.
Pixels each may include a Thin Film Transistor (TFT) that supplies a voltage of a data line to a pixel electrode in response to gate pulses. The gate pulses swing between a Gate High Voltage (VGH) and a Gate Low Voltage (VGL). The VGH is a turn-on voltage of a transistor and is set to a voltage higher than a threshold voltage in an n-type Metal Oxide Semiconductor Field Effect Transistor (MOSFET). The VGH is a turn-off voltage of a transistor and is set to a voltage lower than a threshold voltage in an n-type MOSFET.
Nowadays, technology that houses a gate driver circuit together with a pixel array in a display panel has been applied. Hereinafter, a Gate In Panel (GIP) is a gate driver circuit housed in the display panel. The gate driver circuit includes a shift register. The shift register includes a plurality of stages connected in cascade connection. Stages generate an output in response to a start pulse and shift the output according to a shift clock.
Stages of the shift register include a Q node that charges a gate line, a QB node that discharges a gate line, and a switch circuit connected to the Q node and the QB node. The switch circuit raises a voltage of the gate line by charging the Q node in response to a start pulse or an output of a previous stage and discharges the QB node in response to an output of a next stage or a reset pulse. The switch circuit may be implemented with TFTs of a MOSFET structure.
A touchscreen can be classified into add-on type, on-cell type and in-cell type touchscreens according to the structure thereof. The add-on type touchscreen is configured in such a manner that a display device and a touchscreen are separately manufactured and then the touchscreen is attached to the upper substrate of the display device. The on-cell type touchscreen is constructed in such a manner that elements constituting a touchscreen are directly formed on the surface of an upper glass substrate of a display device. The in-cell type touchscreen can achieve a thin display device by embedding a touchscreen in the display device and improve durability. However, the add-on type touchscreen has problems that the touchscreen is mounted on the display device to increase the thickness of the display device and reduce the brightness of the display device so as to decrease visibility. The on-cell type touchscreen can reduce the thickness of the display device, compared to the add-on type touchscreen, since a separate touchscreen is formed on the surface of the display device but still has problems that the thickness of the display device and the number of manufacturing processes are increased due to driving electrodes and sensing electrodes constituting the touchscreen and insulating layers for insulating the driving electrodes and the sensing electrodes, thereby increasing manufacturing costs.
The in-cell touchscreen can solve the problems of the add-on type and on-cell type touchscreens since durability thereof can be improved and the thickness thereof can be reduced. The in-cell type touchscreen can be classified into an optical touchscreen and a capacitive touchscreen.
The optical touchscreen has a light sensing layer formed on a thin film transistor array of a display device such that light reflected through an object corresponding to a touched point is recognized by using light from a backlight unit or infrared light. However, while the optical touchscreen shows relatively stabilized driving performance in a dark environment, light stronger than reflected light acts as noise in a bright environment. This is because intensity of light reflected by touch is very low and thus touch recognition error may be generated even when surroundings are slightly bright. Particularly, the optical touchscreen has a problem that touch may not be recognized when the surrounding environment is exposed to sunlight due to remarkably high intensity of light.
The capacitive touchscreen can be classified into a self-capacitance type and a mutual capacitance type. The mutual capacitance type touchscreen is configured in such a manner that a common electrode is divided into driving electrodes and sensing electrodes such that mutual capacitance is generated between the driving electrodes and the sensing electrodes to as to recognize touch by measuring mutual capacitance variation generated when touch is applied thereto. However, the mutual capacitance touchscreen cannot correctly recognize a touch point since mutual capacitance generated during touch recognition is very small whereas parasitic capacitance between a gate line and a data line constituting the display device including the touchscreen is very large. In addition, the mutual capacitance touchscreen requires a very complicated interconnection structure because a plurality of touch driving lines for touch operation and a plurality of touch sensing lines for touch sensing need to be formed on common electrodes. To solve this problem, there has been proposed a separate display and touch driving method by which a plurality of electrodes, which is formed to overlap a plurality of pixel electrodes in a display area of a panel, operates as common electrodes for driving liquid crystal along with pixel electrodes respectively formed at pixels for a display time and operates as touch electrodes for sensing touch points according to a touch scan signal supplied from a touch driver circuit for a touch time.
In the separate display and touch driving method, stages constituting a shift register of a gate driver circuit include a stage having a Q node holding in a standby state for touch time. The Q node of the stage is in a floating state in which power is not supplied for the touch time and thus voltage drop due to leakage current occurs. Such problem leads to abnormal signal output to a gate line, resulting in defects such as dim that a horizontal line appears on the display panel corresponding to the gate line. Furthermore, the voltage drop at the Q node of the standby stage increases the touch time.